To form images on a television, an electron beam is scanned across a phosphor screen along a predefined scanning pattern or "raster". As the beam is moved across the screen, its intensity is modulated in accordance with the received analog signal, thereby forming picture elements or pixels on the screen. The amount of illumination at any point on the screen, moreover, is proportional to the beam's intensity at that point. Typically, the scanning pattern comprises a set number of lines. Upon scanning all of the lines of the predefined scanning pattern, a frame corresponding to an image is produced.
To provide sufficient resolution of the image, the U.S. standard provides for 525 lines per frame drawn at a rate of thirty frames per second. This rate, however, results in a sensation of flicker rather than smooth movement. To eliminate flicker, a process known as interlacing is used. More specifically, each frame is divided into two fields of 262.5 lines each and the lines of the two fields are interlaced with one another. By interlacing the lines of the two fields, the screen is effectively refreshed sixty times per second rather than thirty, thereby avoiding the sensation of flicker.
Referring to FIG. 1A, a screen 100 is shown. The scanning pattern utilized in the U.S. starts at the upper left corner of the screen 100 with FIELD1, LINE22. The beam then scans from left to right across the screen 100, modifying the resulting illumination at each point. At the end of each line, the beam is rapidly swept back to the left side of the screen 100 as shown by the dashed horizontal lines 101. This is known as a "horizontal retrace". The beam reaches the bottom middle of the screen 100 at LINE262.5. The beam is then rapidly swept from the bottom middle of the screen 100 up to the top middle of the screen 100 as indicated by dashed vertical line 103. This is known as a "vertical retrace". During the horizontal and vertical retrace periods the electron beam is turned off (i.e., blanked out), since no imaging occurs during these intervals.
From the top middle of the screen 100, the beam resumes imaging with LINE283.5 of FIELD2 and scans through to LINE525 of FIELD2. The beam is then returned to the top left corner of the screen 100 with another vertical retrace 103, thereby completing the scanning pattern. The beam is now ready to begin scanning the next image.
As shown, each vertical retrace comprises twenty-one consecutive lines. That is, LINE1-LINE21 of FIELD1 and LINE263-LINE283 of FIELD2 correspond to the vertical retraces. Accordingly, out of a total of 525 lines per frame, 42 lines are blanked out during vertical retracing. These twenty-one lines per field are known as the vertical blanking interval (VBI) and, as set forth above, unlike the remaining 483 lines of the frame, the lines of the vertical blanking intervals do not carry any imaging information.
FIG. 1B is a schematic illustration of a plurality of lines, including the lines corresponding to the vertical blanking interval 110a and 110b of each field of a frame. As set forth above, each vertical blanking interval 110a, 110b comprises the equivalent of twenty-one horizontal lines of the scanning pattern. None of these lines, however, carries analog television signals since the beam is blanked out during this interval. Furthermore, according to the standards governing television signals in the U.S., lines one through nine of each vertical blanking interval 110a, 110b carry vertical synchronization, pre-equalizing and post-equalizing pulses. That is, these lines are used to synchronize the vertical retracing of the electron beam.
Similarly, each line of the television signal, represented generally by the reference letter H, includes a horizontal synchronizing (H-synch) pulse. The H-synch pulse, which is typically located at the end of the each line, triggers the scanning circuits to blank out the beam and sweep it rapidly back to the left.
As mentioned above, LINE1 though LINE9 of the vertical blanking intervals 110a, 110b are used to carry vertical synchronizing and other information. In addition, LINE21 of the vertical blanking interval 110a corresponding to field one is designated to carry closed captioning information. Closed captioning is a technique used to visually display audio information in text format on a television screen for the hearing impaired. Nevertheless, LINE10 through LINE20 of FIELD1 and LINE10 through LINE21 of FIELD2 are available to carry auxiliary data, referred to as "teletext data". That is, data, such as digital or analog data, may be inserted into these lines without affecting the resulting video signal, since these lines are not used to carry any imaging or other information.
The ability to insert data into the available lines of the vertical blanking interval presents a significant opportunity to transmit data at relatively low cost. First, the infrastructure needed to transmit this data already exists and is in place. No additional transmitting equipment or power is required. The data may simply be inserted into existing transmissions. Second, the inserted data may be accessed by anyone within the broadcast reach of the underlying television signal. Thus, VBI inserted data has the potential of reaching hundreds of thousands of people without any appreciable delay and at little added cost.
Despite these many advantages, current devices for use in inserting data into the vertical blanking interval of television signals are relatively inefficient, inflexible and costly to operate. For example, the TES3 VBI Encoding Platform from Norpak Corporation can only insert data into a single channel. See TES3 product sheet from Norpak Corp. Thus, cable television operators who often carry sixty or more channels must purchase, install and maintain a separate, dedicated TES3 Platform for each channel. This significantly raises the cost and complexity of offering VBI data insertion services.
The TES3, moreover, can only accept data for insertion purposes through a serial communications port, substantially limiting the rate at which data may be inserted into the VBI. In addition, the TES3 is typically pre-configured to insert received data only into the available lines of the vertical blanking interval. Thus, it is extremely difficult, assuming it is even possible, for example, to insert data into something less than all of these available lines should some implementation choose to use less than all of the vertical blanking interval.
It is an object of the present invention to provide an improved VBI insertion device that is capable of inserting data into multiple channels. The insertion device should additionally provide a high level of data throughput. The device should also be preconfigurable to insert data into any preselected lines, such as the lines of the vertical blanking interval or all lines of otherwise unused channels. Furthermore, to provide a high degree of flexibility, the device should be programmable.